The Compactness of Far-Infrared Bright Galaxies

October 1992 • 1992ApJ...398L..33W

Authors • Wang, Zhong • Helou, George

Abstract • The linear size distribution of 218 galaxies in the IRAS infrared bright galaxy sample is studied by examining new measurements of the 1.49 Hz radio continuum emitting regions. Only spatially extended radio emitters are considered so as to exclude known AGNs and suspected nuclear "monster" sources. We find that the radio surface brightness varies over a wide range at a given luminosity, but its median scales approximately as luminosity to the power of 6/5. Consequently the median effective radio size of galaxies in a given luminosity range actually decreases with increasing luminosity, especially among galaxies with L_FIR_> 10^11^ L_sun_. The close correlation between FIR and radio fluxes of these star-forming galaxies then leads to a picture contrary to earlier theoretical predictions that the sizes of FIR emitting regions should increase with luminosity. We propose that such a steep increase of surface brightness in luminous galaxies is a direct result of a high inter-stellar medium density and an enhanced radiation density of the heating photons driven by more active star formation. Subject Headings: galaxies: fundamental parameters infrared: galaxies: - radio continuum: galaxies

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